Why do i have bald spots with hair particlesystem?
For some reason that I cannot find, some of the interpolated child hair strands are growing inwards -no bald areas actually. You can see little dots in those areas which are the root of that bad children that are growing inside the mesh. The interpolation is bad though the face normals are fine and I couldn't find any other cause for this.

So, as the problem is the interpolation between parents, the solution is adding more parents manually. 

Go to the Particle Edit mode, then choose the Add tool and add a parent wherever there is a bald spot. Then, the Children will rearrange around the new parent and will face towards the face normal as the parent.



You can se above that I fixed the right leg.

Note that doing this means have more parents and, eventually, will create more children too. You might want to change the number of children.